Neurotrophic factors are natural proteins, found in the nervous system or in non-nerve tissues innervated by the nervous system, whose function is to promote the survival and maintain the phenotypic differentiation of nerve and/or glial cells (Varon and Bunge 1979 Ann. Rev. Neuroscience 1:327; Thoenen and Edgar 1985 Science 229:238). Because of this physiological role, neurotrophic factors may be useful in treating the degeneration of nerve cells and loss of differentiated function that occurs in a variety of neurodegenerative diseases.
In order for a particular neurotrophic factor to be potentially useful in treating nerve damage, the class or classes of damaged nerve cells must be responsive to the factor. Different neurotrophic factors typically affect distinctly different classes of nerve cells. Therefore, it is advisable to have on hand a variety of different neurotrophic factors to treat each of the classes of damaged neurons that may occur with different forms of disease or injury.
Neurotrophic factors can protect responsive neurons against a variety of unrelated insults. For example, the neurotrophic factor nerve growth factor (NGF) will rescue a significant portion of sensory neurons from death caused by cutting their axonal processes (Rich et al. 1987 J. Neurocytol 16:261; Otto et al. 1987 J. Neurosci 83:156), from ontogenetic death during embryonic development (Hamburger et al. 1984 J. Neurosci 4:767), and from damage caused by administration of taxol or cisplatin (Apfel et al. 1991 Ann Neurol. 29: 87). This apparent generality of protection has lead to the concept that if a neurotrophic factor protects responsive neurons against experimental damage, it may be useful in treating diseases that involve damage to those neurons in patients, even though the etiology may be unknown.
A given neurotrophic factor, in addition to having the correct neuronal specificity, must be available in sufficient quantity to be used as a pharmaceutical treatment. Since neurotrophic factors are typically present in vanishingly small amounts in tissues (e.g., Hofer and Bards 1988 Nature 331:261; Lin et al. 1989 Science 246:1023), it would be inconvenient to prepare pharmaceutical quantities of neurotrophic factors directly from animal tissues. As an alternative, it would be desirable to locate the gene for a neurotrophic factor and use that gene as the basis for establishing a recombinant expression system to produce potentially unlimited amounts of the protein.
The inventors of this application describe a method for screening biological samples for neurotrophic activity on the embryonic precursors of the substantia nigra dopaminergic neurons that degenerate in Parkinson's disease. This bioassay for identifying neurotrophic factors that may be useful in treating Parkinson's disease is based on an assay previously described (Friedman et al. 1987 Neuro. Sci. Lett. 79:65-72, specifically incorporated herein by this reference) and implemented with modifications in the present invention. This assay was used to screen various potential sources for neurotrophic activity directed to dopaminergic neurons. The present invention describes the characterization of a new neurotrophic factor that was purified from one such source, the conditioned culture medium from a glioblastoma cell line, B49 (Schubert et al. 1974 Nature 249:224-27, specifically incorporated herein by this reference). The conditioned medium from this cell line was previously reported to contain dopaminergic neurotrophic activity (Bohn et al. 1989 Soc. Neurosci. Abs. 15:277). In this previous report, the source of the neurotrophic activity was not purified, characterized chemically, or shown to be the consequence of a single agent in the conditioned medium. Nerve damage is caused by conditions that compromise the survival and/or proper function of one or more types of nerve cells. Such nerve damage may occur from a wide variety of different causes, some of which are indicated below.
Nerve damage may occur through physical injury, which causes the degeneration of the axonal processes and/or nerve cell bodies near the site of injury. Nerve damage may also occur because of temporary or permanent cessation of blood flow to parts of the nervous system, as in stroke. Nerve damage may also occur because of intentional or accidental exposure to neurotoxins, such as the cancer and AIDS chemotherapeutic agents cisplatinum and dideoxycytidine (ddC), respectively. Nerve damage may also occur because of chronic metabolic diseases, such as diabetes or renal dysfunction. Nerve damage may also occur because of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and Amyotrophic Lateral Sclerosis (ALS), which result from the degeneration of specific neuronal populations.
This application describes a novel neurotrophic factor. Neurotrophic factors are natural proteins that promote the normal functions of specific nerve cells and/or protect the same cells against a variety of different forms of damage. It is these properties that suggest that GDNF may be useful in treating various forms of nerve damage, including those forms indicated specifically above.
Parkinson's disease is identified by a unique set of symptoms that include rigidity, bradykinesis, seborrhea, festination gait, flexed posture, salivation, and a "pill rolling" tremor. The disease is encountered in all races throughout the world, and the average age of onset is 60 years.
After years of conflicting theories and controversy, a biochemical basis for Parkinson's disease has emerged as the major cause. (See, e.g., Bergman, 1990 Drug Store News, 12:IP19.) Of significant importance to an understanding of Parkinson's disease are the areas of the brain known as the substantia nigra the basal ganglia, and particularly, the corpus striatum. The substantia nigra, a bilaterally paired layer of pigmented gray matter in the mid-brain, is involved with dopamine transmission, while the normal basal ganglia function involves a series of interactions and feedback systems which are associated with the substantia nigra and mediated in part by dopamine, acetylcholine and other substances.
In Parkinson's disease, there is a dysfunction in the dopaminergic activity of the substantia nigra which is caused by neuronal degeneration. This results in a state of dopamine deficiency and a shift in the balance of activity to a cholinergic predominance. Therefore, although there is no increase in the concentration of acetylcholine, the excitatory effects on the central nervous system (i.e., tremors) by this cholinergic mediator overwhelm the inhibiting effects of the depleted dopamine.
The most effective treatment for Parkinson's disease to date is the oral administration of Levodopa. Levodopa penetrates the central nervous system and is enzymatically converted to dopamine in the basal ganglia. It is believed that beneficial effects of Levodopa are, therefore, in increasing the concentration of dopamine in the brain. Unfortunately, neither Levodopa or any of the less commonly utilized medications actually stem the progression of the disease which is caused by the degeneration of dopaminergic neurons.
Other researchers have reported the existence of dopaminergic activity in various biological sources. In PCT publication WO91/01739 of Springer et al., a dopaminergic neurotrophic activity was identified in an extract derived from cells of the peripheral nervous system. The activity identified was not purified, but was attributed to a factor having a molecular weight of less than 10,000 daltons. The factor was isolated from rat sciatic nerve but is apparently not CNTF, which is also found in the nerve (Lin et al. 1989 Science 246:1023).
In U.S. Pat. No. 5,017,735 of Appel et al., dopaminergic activity was identified in an extract from caudate-putamen tissue. Again, no factors giving rise to the activity were purified and the apparent molecular weight of the activity containing fractions was relatively small. See also, Niijima et al. 1990 Brain Res. 528:151-154 (chemically deafferented striatum of adult rat brain); Lo et al. 1990 Soc. Neurosci. Abstr., 16:809 (striatal-derived neurotrophic factor). In addition, other known neurotrophic factors have also been shown to have dopaminergic activity, e.g., Brain derived neurotrophic factor (BDNF), and acidic and basic Fibroblast Growth Factors.
The GNDF of the present invention was isolated based on its ability to promote the functional activity and survival in cell culture of dopaminergic nerve cells isolated from the rat embryo mesencephalon. These dopaminergic nerve cells are the embryonic precursor of the dopaminergic nerve cells in the adult substantia nigra that degenerate in Parkinson's disease. Therefore, GDNF may be useful in reducing the neuronal degeneration that causes the symptoms of Parkinson's disease.
Furthermore, GDNF may be useful in treating other forms of damage to or improper function of dopaminergic nerve cells in human patients. Such damage or malfunction may occur in schizophrenia and other forms of psychosis. Current treatments for such conditions often require drugs active at dopamine receptors, suggesting that improper function of the dopaminergic neurons innervating these receptor-bearing neuronal populations may be involved in the disease process.
Based on previous experience with other neurotrophic factors, new forms of nerve damage that may be treated with GDNF will emerge as more is learned about the various types of nerve cells that are responsive to this neurotrophic factor. For example, nerve growth factor (NGF) only emerged as a potentially useful treatment for Alzheimer's disease when it was recently discovered that NGF acts as a neurotrophic factor for the basal forebrain cholinergic neurons that degenerate in Alzheimer's disease. (Williams, et al. 1986 Proc. Natl. Acad. Sci. USA 83:9231). Methods are provided in the present invention for determining other forms of nerve damage that may be usefully treated with GDNF.
Patrick Aebischer and coworkers have described the use of semipermeable, implantable membrane devices that are useful as means for delivering drugs or medicaments in certain circumstances. For example, they have proposed the encapsulation of cells that secrete neurotransmitter factors, and the implantation of such devices into the brain of patients suffering from Parkinson's Disease. See, U.S. Pat. No. 4,892,538 of Aebischer et al.; U.S. Pat. No. 5,011,472 of Aebischer et al.; U.S. Pat. No. 5,106,627 of Aebisher et al.; PCT Application WO 91/10425; PCT Application WO 91/10470; Winn et al. 1991 Exper. Neurol. 113:322-329; Aebischer et al. 1991 Exper. Neurol. 111:269-275; and Tresco et al. 1992 ASAIO 38:17-23.